Orthogonal Frequency Division Multiplexing (OFDM) subdivides a high data rate input data stream into a number of parallel sub-streams of reduced data rate wherein each sub-stream is modulated and simultaneously transmitted on a separate orthogonal sub-carrier. Referring to FIG. 1, an OFDM transmitter 10 comprises a symbol mapper 16 which groups incoming serial data 14 to form symbols. The symbols are modulated with baseband subcarriers by an inverse DFT 18 and then serialized to form provisional OFDM symbols. A cyclic prefix is formed from a few samples selected from the end of a provisional OFDM symbol. The cyclic prefix is concatenated to the start of the corresponding provisional OFDM symbol. The cyclic prefix and the provisional OFDM symbol, collectively form an OFDM symbol, wherein the cyclic prefix forms the start of the OFDM symbol; and the provisional OFDM symbol forms the rest. The OFDM symbols are then transferred to a DAC 20 in which they are converted to analog form.
Prior to transmitting a first OFDM symbol, the transmitter 10 transmits a special signal, known as a preamble, which is used for synchronization purposes. Thus, an OFDM frame comprises a preamble signal followed by a plurality of OFDM symbols. On receipt of an OFDM frame, an OFDM receiver 24 performs the inverse operations of the OFDM transmitter 10 in reverse order. However, before any receiver algorithms can be employed, the system clock of the receiver 24 must be synchronised with that of the transmitter 10. Symbol timing refers to the process of finding the precise moment when individual OFDM symbols start and end. This moment is used to position the DFT window (i.e. the set of samples used to calculate the DFT of each received OFDM symbol) of the receiver, and thereby demodulate the subcarriers of the received OFDM symbol. Whilst upper layer OFDM protocols (e.g. OFDM Medium Access Control (MAC) policies) provide some rough guidance as to the start of an OFDM symbol, they do not provide an exact indication thereof. In addition, the MAC protocols in a receiver can only operate if the received OFDM symbols have been previously synchronized and decoded; since the synchronization mechanisms at MAC level are more focused on tracking variations in a reference clock signal.
Traditional synchronization approaches rely on the detection of preambles. Referring to FIG. 2, a preamble comprises short OFDM symbols (or preamble symbols) 30, which are used only in the preamble signal. In particular, the preamble only comprises a set of samples obtained from the output of a short IFFT; and does not comprise a cyclic prefix. Preamble symbols are typically shorter than the OFDM symbols used in the rest of an OFDM frame. The use of short preamble symbols minimizes the overhead (on overall transmission efficiency) of transmitting the preamble; and enables simplified implementation of the preamble. The problem of symbol synchronization can be divided into two steps, namely:                timing synchronisation, which involves determining the time shift between transmitted preamble symbols and the receiver DFT window; and        frame synchronisation, which involves determining the start point of the payload (or the last symbol in the preamble) of a received OFDM signal.        
Timing synchronization may be achieved by signal correlation in the time domain (T. M. Schmidl and D.C. Cox, IEEE Trans. On Commun., 1997 (45), 1613-1621) or phase correlation in the frequency domain (i.e. after the DFT operation of the receiver). Phase correlation involves determining the phase shift between the training DFT and the preamble symbols from a cross-correlation peak. The shift can be represented by an angular rotation, wherein the size of the angle provides an indication of the extent of the shift. Phase correlation provides better performance in the presence strong of narrowband-band interferences. In particular, since preamble symbols remain the same during timing synchronization, averaging several symbols allows the DFT window to be aligned even for signals with lower valued (negative) signal to noise ratios (SNR). Once timing synchronization is achieved, the receiver's DFT window is assumed to be aligned with individual preamble symbols.
Frame synchronization successively correlates consecutive preamble symbols to detect the last preamble symbol, wherein at least one of the last preamble symbols is usually sign inverted. This form of correlation can be performed in time domain, (i.e. before the DFT), or in frequency domain (i.e. after the DFT). The correlation process is based on the observation that if two consecutive preamble symbols are exactly the same, a maximum value of correlation is achieved. However, if the symbols are sign inverted, a minimum value of correlation is achieved. Thus, in use, the correlation output is inspected to find abrupt changes therein. However, when the signal is highly corrupted by noise causing negative values of signal to noise ratios (SNR), preamble symbols cannot be processed in this fashion, because the position of the sign inverted symbol is lost (K. Shi, E. Serpedin, IEEE Trans. On Commun., 2004, 3(4), 1271-1284).
A repetitive structure has been included in a preamble by the IEEE802.11a/HyperLAN-II standard (IEEE P802.11a “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: High Speed Physical Layer in the 5 GHz Band”, July 1999 and ETSI DTS/BRAN 030003-1, “Broadband Radio Access Networks (BRAN); HYPERLAN Type 2 Functional Specifications. Part I-Physical (PHY) layer”, June 1999). More particularly, these standards employ a series of short symbols (S) followed by two long symbols (L) to form a preamble such as [S S S S S S S S L L]. WLAN (IEEE802.11a) defines a short symbol as an output of a short FFT (i.e. with a smaller number of points than the FFT used for data symbols) but not comprising a cyclic prefix. Similarly, a long symbol is defined as the output of the same FFT as that used on data symbols, but not comprising a cyclic prefix. Both the long symbols are short symbols are fixed in accordance with the standards.
The long and short symbols are used for fine time/frequency synchronization and for channel estimation. More particularly, the short symbols are used for timing alignment and the long symbols are used for frame synchronization. However, HiperLAN has been designed to work with signals of positive SNR; and it is very difficult to use this approach for synchronizing with signals of lower SNR values because the reliability of the synchronization is highly deteriorated (i.e. there is a very high probability of not correctly synchronizing). Similarly, the HomePlug-AV system (HomePlug PowerLine Alliance, “HomePlug AV baseline specification”, Version 1.1, May. 2007) employs a preamble of the form [S S S S S S S S −S S].
In this case, frame synchronization is achieved by looking for the negative symbol in the preamble. However, these approaches have been designed to work with signals of positive SNR; and are very difficult to use for synchronizing with signals of lower SNR values.